When sheets are fed to a rotary press, proper feeding requires that the sheets first be aligned. In typical feeding systems, sheets are fed onto a feed table, and then conveyed forward to a set of front lay marks, or abutments. If the front edge of the sheet is properly aligned on the front lay marks, the sheets are then traversely conveyed to a side lay mark, or lateral abutment, so that the left edge of the sheet (or alternatively the right edge) becomes correctly aligned. Once the front and side edges are in proper alignment, the lay marks are moved out of the way and the sheet is then supplied to the press. One such system is described in German patent specification 1,561,015. While the various forms of mechanical side lay mechanisms are known in the art, to the extent necessary the disclosure of said German specification is hereby incorporated by reference.
The known means for detecting if a sheet is properly on the lay marks is to use sensors located near the lay marks, so that when a sheet should be in alignment, a control unit reads the appropriate sensor or sensors and thereby evaluates the sheet position. Further steps in the sheet feeding procedure, (e.g., transverse conveyance) depend on the outcome of the evaluations.
Typically, the sensors are optical reflection sensors which operate in a known manner to detect the presence or absence of a sheet. For example, the sensors operate by optical reflection so that when a sheet passes underneath a light source the sheet reflects light to the sensor, thereby indicating its presence.
In addition to sensing for proper alignment, a sheet can also be sensed earlier, during its forward conveyance, (i.e., toward the front lay marks) to ensure that the sheet will not interfere with the returning lateral abutment. For example, if the sheet is so far left that its left edge is moving below the abutment, the return of the abutment will jam the sheet. This condition is known as excess draw, or draw-over.
The conventional way to monitor for draw-over is to position a first lateral sensor on the outside of the lateral abutment, that is, at a location where the side edge of the sheet should not extend beyond during its forward conveyance. If this first lateral detector detects a sheet, the control unit takes corrective measures such as halting the drive to prevent improper sheet conveyance.
In these conventional systems, a second sensor is employed to detect and control proper side lay mark alignment. Thus a second lateral sensor is located inside the side abutment to detect proper side edge alignment. This is not the same lateral sensor that monitors for draw-over of the sheet, but a sensor for side lay mark monitoring. When this second sensor detects the presence of a sheet, it actuates the transverse conveyor to convey the sheet toward the lateral side lay abutment. The front sheet sensors are thereafter actuated by the leading edge of the sheet to signal that the sheet has been properly fed.
However, this known system of side lay mark monitoring suffers from a particular problem when the sheet is too near the lateral abutment after forward conveyance, although not far enough to have been in a draw-over condition. When this occurs, the sheet cannot make any appreciable lateral movement. Often, however, some lateral movement is necessary to correct alignment of a slightly skewed sheet. Thus, if a slightly skewed sheet is too close to the lateral abutment, deformation (crushing) of the sheet occurs because the transverse conveyor applies a force to the sheet at a time when the sheet cannot move far enough in the transverse direction to correct itself.